Bring your ID, the VIN for the car and a credit card with lots of space on it. A transponder key will cost big bucks, and most cars won't even start unless the transponder is working properly. A mechanic can't duplicate the key, either. It has to be ordered from the dealership or from a certified locksmith.
One way to help minimize the problem is to get a key made from the regular key at the hardware store. This won't start the engine, *but* if you lock your keys in the car, it will open the door so you can get them out.
Some cars won't let you lock your keys in. They're smart enough to know if the keys are in the ignition and the doors are locked, but the engine isn't running. My car doors will unlock automatically after five minutes.
The other advantage of having a regular key that will unlock the door is, if the transponder keys are lost, a dealer will still have a key that will unlock the door, which enables them to order a transponder key and fob, rather than having the locks on the doors changed, which costs a lot more than a key.